The present invention generally relates to wireless communication and particularly relates to polar modulation methods and apparatus.
Current and evolving wireless communication standards, such as WCDMA (Wideband CDMA), EDGE (Enhanced Data Rates through GSM Evolution), and GSM (Global System for Mobile communications), combine a variety of digital modulation formats. Such formats include 8-QPSK modulation, various QAM implementations, GMSK modulation, etc. Of the several aims embodied in these current and developing wireless standards are increased data rates and increased spectral efficiency.
Achieving such aims involves, in part, the use of relatively complex digital modulation formats, which impose stringent linearity requirements for transmit signal generation. Biasing transmitter power amplifiers to linear operating points may satisfy amplifier linearity requirements, but does so at the expense of power efficiency. Such power inefficiency runs at cross-purposes to achieving low overall operating power and, in the context of portable communication devices, severely limits battery life.
Polar modulation transmitters stand as alternative to transmitters based on conventional linear amplifiers. Polar modulation splits the transmit information stream into coordinated streams of phase and amplitude information. The phase information modulates a radio frequency carrier signal having a constant signal envelope, and a saturated power amplifier receives this phase-modulated signal as its input signal. An amplitude modulation controller, such as a controlled voltage or current source modulates the supply power to the saturated power amplifier and thereby imparts amplitude modulation to the amplifier's output signal. Thus, polar modulation techniques enable linear amplitude modulation while allowing the use of power-efficient saturated power amplifiers.
Polar modulation offers additional benefits, such as enabling generic transmitter architectures by obviating the need for the band-pass filtering elements typically needed by conventional linear transmitter chains. Thus, polar modulation transmitters may, if properly configured, be used for a variety of modulation formats, and thereby offer the opportunity to use essentially the same transmitter architecture for GSM, EDGE, and WCDMA standards, among others.
However, as with most technical innovations, polar modulation has certain disadvantages or, more properly stated, certain limitations, that present challenges to its use. Such challenges involve the expansion of phase and amplitude bandwidths associated with “unwrapping” the phase and amplitude information. For example, the phase modulation bandwidths become fairly high, particularly in wideband systems, such as WCDMA. As such, polar modulation requirements may not be easily met with conventional phase modulation circuits, such as narrow-band Phase Locked Loops (PLLs).
Additionally, the relatively wide ranging transmit-power control required by, for example, the WCDMA standards, poses challenges to polar modulation transmitters. Further, splitting transmitter phase and amplitude information onto separate transmitter paths introduces the potential for group delay disparities between the phase and amplitude paths. In other words, timing differences in the phase and amplitude paths introduce relative time shifts between the phase and amplitude modulation information streams, resulting in potentially severe distortion in the final radio frequency (RF) transmit signal.
Such distortion may be measured in terms of an Error Vector Magnitude (EVM), which is a time domain representation of disparity between the nominal and actual transmit signal waveforms. Other parameters of interest include Adjacent Channel Power Ratio (ACPR), which is a measure of main channel power to unwanted signal spread into adjacent frequency channels. Transmit signal non-linearity gives rise to significant ACPR and thus transmitter chain linearity must be tightly controlled.
Acceptable EVM and ACPR performance depends on correct time alignment between the phase modulation and amplitude modulation envelopes within a polar modulation transmitter. For example, EDGE-based systems typically can tolerate no more than 0.1 symbols of time misalignment between the polar and amplitude modulation envelopes. No more than 0.1 “chips” of time misalignment typically can be tolerated for WCDMA-based systems. With a chipping rate of 3.84 Mega-chips-per-second (Mcps), one readily appreciates the stringency of such a requirement.
Thus, an ideal polar modulation transmitter would offer wideband signal capability. Additionally, the ideal polar modulation transmitter would include provisions for monitoring and controlling relative path delays for the amplitude and phase modulation envelopes for tight control of the transmit signal in terms of ACPR and EVM requirements, for example.